The present invention relates generally to a packing material for aseptic packages. More specifically, the present invention relates to a web of packing material which is provided with an easily separable coating layer for maintaining one surface of the packing material in a sterile condition.
The invention, moreover, relates to the method for the manufacture of the packing material and the use of the packing material in the manufacture of packing containers.
It is known that so-called aseptic packages of the non-returnable type can be manufactured by the filling of sterile contents into sterilized packing containers, the filling process having to be carried out, of course, under aseptic conditions. One example of such a packing method is the aseptic packing system marketed by Tetra Pak International AB wherein a web of a packing material, consisting of a laminate of paper and plastics and frequently also aluminium foil, is formed to a tube with interior plastic coating (usually polyethylene) by joining together the longitudinal edges of the web to form a tube which is filled and separated into individual packing containers. The inside of the packing material after tube formation has to be sterilized, so that the sterile contents should keep their sterility in the package, and this is done by heating of the inner plastic layer of the tube with the help of a heater introduced into the tube or by a chemical process, whereby the plastic inside is put into contact with a sterilizing agent, preferably hydrogen peroxide. The most common process is, however, that a combination of chemical and thermal sterilization is used. In such a process the web is brought into contact first with hydrogen peroxide by being dipped into a bath and the tube formed is subsequently heated by a heater introduced into the tube, so that on the one hand the hydrogen peroxide decomposes and vanishes and on the other hand the inside of the tube is fully sterilized.
In order to achieve and to maintain full sterility of the packing material web, the tube forming process must take place inside a closed sterile chamber, wherein an aseptic atmosphere is maintained under a slight pressure. Also the sterilization process must be accurately monitored so as to ensure in a reliable manner the complete sterilization of the inside of the material web.
The plastic inside of the packing material is actually sterile when the plastic coating is applied with the help of an extrusion process, since the plastic in the coating operation has a temperature of approx. 200.degree. C., that is a temperature which substantially exceeds the temperature at which bacteria and microorganisms can stay alive. However, immediately after cooling, the packing material produced comes into contact with air contaminated by bacteria, so that the plastic coating, sterile at the moment of manufacture, is infected. Thus, the plastic surface of the packing material, which is intended to form the inside of the packing container produced, has to be sterilized when the packaging takes place. This sterilization of the packing material web with the help of thermal and/or chemical agents can be avoided, though, if the plastic layer of the packing material web is provided with a thin protective coating of a non-porous, bacteria-tight plastics which has such good adhesion to the plastic layer of the packing material web that its sterility is maintained. The protective plastic at the same time does not actually fuse together with the plastic layer of the packing material, but can be pulled off the same when the packaging takes place, thus exposing the sterile plastic coating of the packing material. Therefore the packing material is provided along the whole side, which is intended to form the inside of the packages with a bacteriatight, relatively easily separable, thin plastic coating, and the side of the packing material, which is intended to form the inside of the package, as well as the inside of the thin plastic coating, are sterile.